![Jiri Slaby](/assets/img/avatar_default.png)
As the data parameter is not really used by any ftrace_dyn_arch_init, remove that from ftrace_dyn_arch_init. This also removes the addr local variable from ftrace_init which is now unused. Note the documentation was imprecise as it did not suggest to set (*data) to 0. Link: http://lkml.kernel.org/r/1393268401-24379-4-git-send-email-jslaby@suse.cz Cc: Frederic Weisbecker <fweisbec@gmail.com> Cc: Ingo Molnar <mingo@redhat.com> Cc: linux-arch@vger.kernel.org Signed-off-by: Jiri Slaby <jslaby@suse.cz> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
409 lines
15 KiB
Plaintext
409 lines
15 KiB
Plaintext
function tracer guts
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====================
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By Mike Frysinger
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Introduction
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------------
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Here we will cover the architecture pieces that the common function tracing
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code relies on for proper functioning. Things are broken down into increasing
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complexity so that you can start simple and at least get basic functionality.
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Note that this focuses on architecture implementation details only. If you
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want more explanation of a feature in terms of common code, review the common
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ftrace.txt file.
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Ideally, everyone who wishes to retain performance while supporting tracing in
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their kernel should make it all the way to dynamic ftrace support.
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Prerequisites
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-------------
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Ftrace relies on these features being implemented:
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STACKTRACE_SUPPORT - implement save_stack_trace()
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TRACE_IRQFLAGS_SUPPORT - implement include/asm/irqflags.h
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HAVE_FUNCTION_TRACER
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--------------------
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You will need to implement the mcount and the ftrace_stub functions.
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The exact mcount symbol name will depend on your toolchain. Some call it
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"mcount", "_mcount", or even "__mcount". You can probably figure it out by
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running something like:
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$ echo 'main(){}' | gcc -x c -S -o - - -pg | grep mcount
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call mcount
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We'll make the assumption below that the symbol is "mcount" just to keep things
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nice and simple in the examples.
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Keep in mind that the ABI that is in effect inside of the mcount function is
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*highly* architecture/toolchain specific. We cannot help you in this regard,
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sorry. Dig up some old documentation and/or find someone more familiar than
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you to bang ideas off of. Typically, register usage (argument/scratch/etc...)
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is a major issue at this point, especially in relation to the location of the
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mcount call (before/after function prologue). You might also want to look at
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how glibc has implemented the mcount function for your architecture. It might
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be (semi-)relevant.
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The mcount function should check the function pointer ftrace_trace_function
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to see if it is set to ftrace_stub. If it is, there is nothing for you to do,
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so return immediately. If it isn't, then call that function in the same way
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the mcount function normally calls __mcount_internal -- the first argument is
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the "frompc" while the second argument is the "selfpc" (adjusted to remove the
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size of the mcount call that is embedded in the function).
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For example, if the function foo() calls bar(), when the bar() function calls
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mcount(), the arguments mcount() will pass to the tracer are:
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"frompc" - the address bar() will use to return to foo()
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"selfpc" - the address bar() (with mcount() size adjustment)
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Also keep in mind that this mcount function will be called *a lot*, so
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optimizing for the default case of no tracer will help the smooth running of
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your system when tracing is disabled. So the start of the mcount function is
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typically the bare minimum with checking things before returning. That also
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means the code flow should usually be kept linear (i.e. no branching in the nop
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case). This is of course an optimization and not a hard requirement.
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Here is some pseudo code that should help (these functions should actually be
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implemented in assembly):
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void ftrace_stub(void)
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{
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return;
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}
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void mcount(void)
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{
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/* save any bare state needed in order to do initial checking */
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extern void (*ftrace_trace_function)(unsigned long, unsigned long);
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if (ftrace_trace_function != ftrace_stub)
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goto do_trace;
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/* restore any bare state */
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return;
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do_trace:
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/* save all state needed by the ABI (see paragraph above) */
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unsigned long frompc = ...;
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unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
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ftrace_trace_function(frompc, selfpc);
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/* restore all state needed by the ABI */
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}
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Don't forget to export mcount for modules !
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extern void mcount(void);
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EXPORT_SYMBOL(mcount);
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HAVE_FUNCTION_TRACE_MCOUNT_TEST
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-------------------------------
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This is an optional optimization for the normal case when tracing is turned off
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in the system. If you do not enable this Kconfig option, the common ftrace
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code will take care of doing the checking for you.
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To support this feature, you only need to check the function_trace_stop
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variable in the mcount function. If it is non-zero, there is no tracing to be
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done at all, so you can return.
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This additional pseudo code would simply be:
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void mcount(void)
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{
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/* save any bare state needed in order to do initial checking */
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+ if (function_trace_stop)
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+ return;
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extern void (*ftrace_trace_function)(unsigned long, unsigned long);
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if (ftrace_trace_function != ftrace_stub)
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...
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HAVE_FUNCTION_GRAPH_TRACER
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--------------------------
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Deep breath ... time to do some real work. Here you will need to update the
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mcount function to check ftrace graph function pointers, as well as implement
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some functions to save (hijack) and restore the return address.
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The mcount function should check the function pointers ftrace_graph_return
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(compare to ftrace_stub) and ftrace_graph_entry (compare to
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ftrace_graph_entry_stub). If either of those is not set to the relevant stub
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function, call the arch-specific function ftrace_graph_caller which in turn
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calls the arch-specific function prepare_ftrace_return. Neither of these
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function names is strictly required, but you should use them anyway to stay
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consistent across the architecture ports -- easier to compare & contrast
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things.
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The arguments to prepare_ftrace_return are slightly different than what are
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passed to ftrace_trace_function. The second argument "selfpc" is the same,
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but the first argument should be a pointer to the "frompc". Typically this is
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located on the stack. This allows the function to hijack the return address
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temporarily to have it point to the arch-specific function return_to_handler.
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That function will simply call the common ftrace_return_to_handler function and
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that will return the original return address with which you can return to the
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original call site.
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Here is the updated mcount pseudo code:
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void mcount(void)
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{
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...
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if (ftrace_trace_function != ftrace_stub)
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goto do_trace;
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+#ifdef CONFIG_FUNCTION_GRAPH_TRACER
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+ extern void (*ftrace_graph_return)(...);
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+ extern void (*ftrace_graph_entry)(...);
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+ if (ftrace_graph_return != ftrace_stub ||
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+ ftrace_graph_entry != ftrace_graph_entry_stub)
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+ ftrace_graph_caller();
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+#endif
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/* restore any bare state */
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...
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Here is the pseudo code for the new ftrace_graph_caller assembly function:
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#ifdef CONFIG_FUNCTION_GRAPH_TRACER
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void ftrace_graph_caller(void)
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{
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/* save all state needed by the ABI */
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unsigned long *frompc = &...;
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unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
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/* passing frame pointer up is optional -- see below */
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prepare_ftrace_return(frompc, selfpc, frame_pointer);
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/* restore all state needed by the ABI */
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}
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#endif
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For information on how to implement prepare_ftrace_return(), simply look at the
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x86 version (the frame pointer passing is optional; see the next section for
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more information). The only architecture-specific piece in it is the setup of
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the fault recovery table (the asm(...) code). The rest should be the same
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across architectures.
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Here is the pseudo code for the new return_to_handler assembly function. Note
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that the ABI that applies here is different from what applies to the mcount
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code. Since you are returning from a function (after the epilogue), you might
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be able to skimp on things saved/restored (usually just registers used to pass
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return values).
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#ifdef CONFIG_FUNCTION_GRAPH_TRACER
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void return_to_handler(void)
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{
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/* save all state needed by the ABI (see paragraph above) */
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void (*original_return_point)(void) = ftrace_return_to_handler();
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/* restore all state needed by the ABI */
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/* this is usually either a return or a jump */
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original_return_point();
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}
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#endif
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HAVE_FUNCTION_GRAPH_FP_TEST
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---------------------------
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An arch may pass in a unique value (frame pointer) to both the entering and
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exiting of a function. On exit, the value is compared and if it does not
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match, then it will panic the kernel. This is largely a sanity check for bad
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code generation with gcc. If gcc for your port sanely updates the frame
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pointer under different optimization levels, then ignore this option.
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However, adding support for it isn't terribly difficult. In your assembly code
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that calls prepare_ftrace_return(), pass the frame pointer as the 3rd argument.
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Then in the C version of that function, do what the x86 port does and pass it
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along to ftrace_push_return_trace() instead of a stub value of 0.
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Similarly, when you call ftrace_return_to_handler(), pass it the frame pointer.
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HAVE_FTRACE_NMI_ENTER
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---------------------
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If you can't trace NMI functions, then skip this option.
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<details to be filled>
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HAVE_SYSCALL_TRACEPOINTS
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------------------------
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You need very few things to get the syscalls tracing in an arch.
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- Support HAVE_ARCH_TRACEHOOK (see arch/Kconfig).
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- Have a NR_syscalls variable in <asm/unistd.h> that provides the number
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of syscalls supported by the arch.
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- Support the TIF_SYSCALL_TRACEPOINT thread flags.
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- Put the trace_sys_enter() and trace_sys_exit() tracepoints calls from ptrace
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in the ptrace syscalls tracing path.
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- If the system call table on this arch is more complicated than a simple array
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of addresses of the system calls, implement an arch_syscall_addr to return
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the address of a given system call.
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- If the symbol names of the system calls do not match the function names on
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this arch, define ARCH_HAS_SYSCALL_MATCH_SYM_NAME in asm/ftrace.h and
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implement arch_syscall_match_sym_name with the appropriate logic to return
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true if the function name corresponds with the symbol name.
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- Tag this arch as HAVE_SYSCALL_TRACEPOINTS.
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HAVE_FTRACE_MCOUNT_RECORD
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-------------------------
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See scripts/recordmcount.pl for more info. Just fill in the arch-specific
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details for how to locate the addresses of mcount call sites via objdump.
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This option doesn't make much sense without also implementing dynamic ftrace.
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HAVE_DYNAMIC_FTRACE
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-------------------
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You will first need HAVE_FTRACE_MCOUNT_RECORD and HAVE_FUNCTION_TRACER, so
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scroll your reader back up if you got over eager.
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Once those are out of the way, you will need to implement:
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- asm/ftrace.h:
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- MCOUNT_ADDR
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- ftrace_call_adjust()
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- struct dyn_arch_ftrace{}
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- asm code:
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- mcount() (new stub)
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- ftrace_caller()
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- ftrace_call()
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- ftrace_stub()
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- C code:
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- ftrace_dyn_arch_init()
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- ftrace_make_nop()
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- ftrace_make_call()
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- ftrace_update_ftrace_func()
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First you will need to fill out some arch details in your asm/ftrace.h.
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Define MCOUNT_ADDR as the address of your mcount symbol similar to:
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#define MCOUNT_ADDR ((unsigned long)mcount)
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Since no one else will have a decl for that function, you will need to:
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extern void mcount(void);
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You will also need the helper function ftrace_call_adjust(). Most people
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will be able to stub it out like so:
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static inline unsigned long ftrace_call_adjust(unsigned long addr)
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{
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return addr;
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}
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<details to be filled>
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Lastly you will need the custom dyn_arch_ftrace structure. If you need
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some extra state when runtime patching arbitrary call sites, this is the
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place. For now though, create an empty struct:
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struct dyn_arch_ftrace {
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/* No extra data needed */
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};
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With the header out of the way, we can fill out the assembly code. While we
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did already create a mcount() function earlier, dynamic ftrace only wants a
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stub function. This is because the mcount() will only be used during boot
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and then all references to it will be patched out never to return. Instead,
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the guts of the old mcount() will be used to create a new ftrace_caller()
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function. Because the two are hard to merge, it will most likely be a lot
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easier to have two separate definitions split up by #ifdefs. Same goes for
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the ftrace_stub() as that will now be inlined in ftrace_caller().
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Before we get confused anymore, let's check out some pseudo code so you can
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implement your own stuff in assembly:
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void mcount(void)
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{
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return;
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}
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void ftrace_caller(void)
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{
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/* implement HAVE_FUNCTION_TRACE_MCOUNT_TEST if you desire */
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/* save all state needed by the ABI (see paragraph above) */
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unsigned long frompc = ...;
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unsigned long selfpc = <return address> - MCOUNT_INSN_SIZE;
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ftrace_call:
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ftrace_stub(frompc, selfpc);
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/* restore all state needed by the ABI */
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ftrace_stub:
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return;
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}
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This might look a little odd at first, but keep in mind that we will be runtime
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patching multiple things. First, only functions that we actually want to trace
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will be patched to call ftrace_caller(). Second, since we only have one tracer
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active at a time, we will patch the ftrace_caller() function itself to call the
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specific tracer in question. That is the point of the ftrace_call label.
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With that in mind, let's move on to the C code that will actually be doing the
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runtime patching. You'll need a little knowledge of your arch's opcodes in
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order to make it through the next section.
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Every arch has an init callback function. If you need to do something early on
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to initialize some state, this is the time to do that. Otherwise, this simple
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function below should be sufficient for most people:
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int __init ftrace_dyn_arch_init(void)
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{
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return 0;
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}
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There are two functions that are used to do runtime patching of arbitrary
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functions. The first is used to turn the mcount call site into a nop (which
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is what helps us retain runtime performance when not tracing). The second is
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used to turn the mcount call site into a call to an arbitrary location (but
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typically that is ftracer_caller()). See the general function definition in
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linux/ftrace.h for the functions:
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ftrace_make_nop()
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ftrace_make_call()
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The rec->ip value is the address of the mcount call site that was collected
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by the scripts/recordmcount.pl during build time.
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The last function is used to do runtime patching of the active tracer. This
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will be modifying the assembly code at the location of the ftrace_call symbol
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inside of the ftrace_caller() function. So you should have sufficient padding
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at that location to support the new function calls you'll be inserting. Some
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people will be using a "call" type instruction while others will be using a
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"branch" type instruction. Specifically, the function is:
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ftrace_update_ftrace_func()
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HAVE_DYNAMIC_FTRACE + HAVE_FUNCTION_GRAPH_TRACER
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------------------------------------------------
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The function grapher needs a few tweaks in order to work with dynamic ftrace.
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Basically, you will need to:
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- update:
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- ftrace_caller()
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- ftrace_graph_call()
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- ftrace_graph_caller()
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- implement:
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- ftrace_enable_ftrace_graph_caller()
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- ftrace_disable_ftrace_graph_caller()
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<details to be filled>
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Quick notes:
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- add a nop stub after the ftrace_call location named ftrace_graph_call;
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stub needs to be large enough to support a call to ftrace_graph_caller()
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- update ftrace_graph_caller() to work with being called by the new
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ftrace_caller() since some semantics may have changed
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- ftrace_enable_ftrace_graph_caller() will runtime patch the
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ftrace_graph_call location with a call to ftrace_graph_caller()
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- ftrace_disable_ftrace_graph_caller() will runtime patch the
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ftrace_graph_call location with nops
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